Ventricular assistive devices (VADs) are mechanical pumps that compensate for damaged or otherwise impaired hearts. They are used to restore normal hemodynamics and end-organ blood flow.
Early VADs closely emulated the pumping mechanism of the heart. Such devices provided a chamber which blood could be drawn into and then expelled out of. In the Thermo Cardiosystems pump, blood was drawn in and pushed out by means of a pusher plate mechanism. (Catanese et al., xe2x80x9cOutpatient Left Ventricular Assist Device Support: A Destination Rather Than A Bridge,xe2x80x9d Ann. Thoracic Surg., 62:646-53, 1996.)
There are several problems with such devices. First, the only controllable quantity is the speed of the electric motor, which determines the number of pressure pulses per minute and the total volume of blood flow. Thus, it is not easy to tailor such devices for the needs of particular patients or particular circumstances. Second, the devices are inefficient, because the initial tendency of the pusher plate is to push the blood in all directions. Although the blood flow is eventually confined to only one direction by the action of the valves, it takes energy to actuate the output valve and to overcome the initially diffused motion.
A significant improvement in VADs is achieved by replacing impeller pumps with axial flow pumps, such as the Nimbus pump. (U.S. Pat. No. 5,588,812) Instead of using a pusher plate, axial flow pumps generally use blades or fins attached to a pump rotor to propel blood axially along a cylindrical conduit. However, blood has substantial viscosity and tends to also move radially with the propelling blades or fins. At a rotational speed of several thousand revolutions per minute, the centrifugal force cannot be ignored. As the centrifugally moving blood impinges against the walls of the cylindrical conduit, there is not only a loss of energy but also damage to the blood cells.
The invention herein is a ventricular assistive device based on a progressive cavity pump. A progressive cavity pump does not use blades or fins to propel the blood. Instead, the pump stator and rotor are designed so that, when combined, there are a series of cavities formed between the pump rotor and the stator wall. Blood is carried through the pump chamber in these cavities when the rotor (or the rotor and stator both, as described below) rotates. In the preferred embodiment of the invention, the cavities progress on a straight line path through the pump, providing an unobstructed channel for blood flow and minimizing the risk of thrombus, i.e., blood clotting. Thus, the invention provides a more efficient mechanism for transporting blood that also reduces damage to the blood cells and reduces the risk of thrombus.